I'd argue that it diverges not only across different users, but is also variable over time - as other tools, customer expectations, user expectations, paradigms

I'd argue that it diverges not only across different users, but is also variable over time - as other tools, customer expectations, user expectations, paradigms of interfaces and human-computer interactions change, there is some amount of new features and changes which needs to be done not to lose touch. Of course if we could isolate the whole domain the application fits in from external influence there might be a stable optimum feature set, but I don't think that's how it works in practice.